1. Field of the Invention
The present invention relates to a method of continuously and noninvasively determining a ratio of concentrations of a plurality of light-absorbing materials in blood, particularly in arterial blood, of a living tissue. This method is used mainly in the medical field.
2. Related Art
The principle of this kind of technique is called a pulse photometry. The pulse photometry will briefly be described hereunder.
Arterial blood is present in living tissue. An amount of the arterial blood in the tissue periodically varies with periodic contraction of the heart. Accordingly, when light of a constant intensity is irradiated into living tissue, light transmitted through the living tissue periodically varies in intensity in a pulsation. The amplitude of a pulsation waveform of the light intensity variation reflects a light absorbing characteristic Of each of a plurality of components contained in the arterial blood.
Therefore, a ratio of concentrations of a plurality of light-absorbing materials in the arterial blood is able to be obtained in a manner that a plurality of light rays of proper wavelengths are irradiated into living tissue and lights transmitted through are measured, and the measured values are appropriately processed. This is the principle of the photometry.
This kind of the concentration-ratio measuring method was first described in Japanese patent Unexamined Japanese Patent Publication Sho 50-128387. Two specific applications of the technique are described in the specification of the patent.
One of those applications is a method of continuously and nonivasively measuring an oxygen saturation of hemoglobin in arterial blood. This measuring method is called a pulse oximetry. An apparatus based on this method is called a pulse oximeter. The apparatus is widely used in the present medical field, and indispensably used, for example, in surgical operations on patients for securing their safety.
The other application is a method of measuring a dye dilution curve. In the measurement, dye is injected into a blood vessel of the human body, and a concentration of the dye in the arterial blood is noninvasively and continuously measured. By the measuring method, the cardiac output, circulatory blood volume, effective hepatic blood flow, and the like may easily be measured. An apparatus based on the measuring method is also put into practice, and commercially available.
The conventional applications of the pulse photometry are as stated above. Those applications have the following problems to be solved, however.
(1) Improvement of a level of measuring accuracy of the conventional apparatus:
With regard to the pulse oximeter, in the oxygen inhalation for a premature baby suffering from respiratory insufficiency, for example, it is required that a concentration of oxygen inhaled is as low as possible. However, the present pulse oximeter is insufficient in its measuring accuracy for adjusting a concentration of inhaled oxygen to a proper concentration value. With regard to the dye dilution curve measuring apparatus, its measuring accuracy must be high in order that a doctor quickly and accurately diagnoses in clinical diagnosis.
(2) To pursue the possibility of a variety of applications
In an example where a trace of carboxyhemoglobin or methemoglobin is contained in the arterial blood or when a trace of bilirubin is contained therein, it is important to nonivasively and accurately measure it. Such a measurement is impossible at the present stage, however. The measurement of the dye dilution curve is greatly affected by a variation of an oxygen saturation when some kind of dye is used Those problems must be solved. The number of wavelengths of lights must be increased to solve those problems.
So far as we know, there is no method of correctly carrying out the measurement in question by increasing the number of wavelengths method of lights, however. This is the problem of multiple wavelengths. The present invention presents a solution to this problem.
The present invention is made to present a solution to the multiple wavelength problem in the pulse photometry.
According to one aspect of the invention, there is provided a probe having a light irradiating device for irradiating light to living tissue and a light receiving device for receiving light from the living tissue, wherein the light irradiating device includes a light source for emitting a plurality of lights of different wavelengths, and a first light scattering portion located in front of the light source, and the light receiving device includes a photo-electric transducing portion for producing a signal based on an intensity of light received on a light sensitive surface, and a second light scattering portion.
According to another aspect of the invention, the light receiving device includes a light mixing portion provided between the living tissue and the light sensitive surface.
According to a third aspect of the invention, the light mixing portion includes a closed space a part of the inner wall of which includes the transmitted side surface of the second light scattering portion and the light sensitive surface.
According to a fourth aspect of the invention, the first light scattering portion consists of a light scattering plate.
According to a fifth aspect of the invention, the second light scattering potion includes a light scattering plate or a thing which scatters light by a light reflecting surface thereof.
According to a further aspect of the invention, there is provided an apparatus for determining concentrations of light-absorbing materials in living tissue, comprising: the probe; and concentration-ratio processing means for computing a ratio of concentrations of a plurality of light-absorbing materials in the living tissue based on an output signal of the photo-electric transducing portion of the probe.
According to an additional aspect of the invention, the concentration-ratio processing means obtains a variation of an optical attenuation of the living tissue based on a pulsating component of an output signal of the photo-electric transducing portion, and computes a ratio of concentrations of a plurality of light-absorbing materials based on the obtained attenuation variation.
According to another aspect of the invention, the concentration-ratio processing means includes attenuation variation component detecting means for obtaining attenuation variation components xcex94A1, xcex94A2, . . . , xcex94An of the respective wavelengths from variations of lights transmitted through or reflected by the living tissue when the living tissue is irradiated by the light irradiating device, variation component ratio detecting means for obtaining a ratio xcfx86ij of each of an xe2x80x9cmxe2x80x9d number of combinations of two attenuation variation components (xcex94Ai, xcex94Aj) selected from an xe2x80x9cnxe2x80x9d number of attenuation variation components xcex94A1, xcex94A2, . . . , xcex94An obtained by the attenuation variation component detecting means, and computing means for computing at least one of an oxygen saturation and a ratio of concentrations of other light-absorbing materials in blood based upon an xe2x80x9cmxe2x80x9d number of simultaneous equations about the respective wavelengths and an xe2x80x9cmxe2x80x9d number of ratios xcfx86ij obtained by the variation component ratio detecting means, on the assumption that the attenuation variation component is the sum of the attenuation variation components of absorbing attenuation and non-absorbing attenuation.
According to another aspect of the invention, the photo-electric transuding portion and the second light scattering portion are confronted from each other.